A significant detriment to the energy density of most batteries is posed by the battery's cathode. This is true for battery chemistries using, for example, lithium or nickel. Typically, oxidant is stored at the cathode at a molar charge capacity that is two to five times lower than that of the anode. Many fuel cells, on the other hand, use oxygen from the air as a source of oxidant. The existence of a continuous and virtually limitless oxidant source enables, in principle, high energy density. However, the use of hydrogen and organic fuels precludes high energy efficiencies due to problems with vapor pressure and balance-of-systems complexity, such as humidification and membrane issues. Metal-air electrochemical cells are able to combine the ultra-high anode capacity of batteries with the air-breathing cathode of fuel cells in order to achieve substantial energy densities that are relevant to modern energy demands.
Metal-air batteries typically include a fuel electrode at which metal fuel is oxidized, an air electrode at which oxygen is reduced, and an electrolyte for providing ion conductivity. A limiting factor with conventional metal-air batteries is the evaporation of the electrolyte solution (i.e., the ionically conductive medium), particularly the evaporation of the solvent, such as water in an aqueous electrolyte solution. Because the air electrode is required to be air permeable to absorb oxygen, it also may permit the solvent vapor, such as water vapor, to escape from the cell. Over time, the cell becomes incapable of operating effectively because of the depletion of the solvent. Indeed, in many cell designs this evaporation issue renders the cell inoperable before the fuel is consumed. The evaporation issue is exacerbated in secondary (i.e., rechargeable) cells, because the fuel may be re-charged repeatedly over the life of the cell, whereas the electrolyte solution cannot (absent replenishment from an external source).
There are other problems associated with conventional aqueous electrolyte batteries, such as water electrolysis during recharging, and self discharge. During recharge, a current is passed through the battery to reduce the oxidized fuel at the fuel electrode. Some of the current, however, electrolyzes the water resulting in hydrogen evolution (reduction) at the fuel electrode and oxygen evolution (oxidation) at the oxygen electrode as represented in the following equations:Reduction: 2H2O(l)+2e−→H2(g)+2OH−(aq)  (1)andOxidation: 2H2O(l)→O2(g)+4H+(aq)+4e−  (2)In this manner, further aqueous electrolyte is lost from the battery. Additionally, the electrons that are consumed in reducing hydrogen are not available to reduce the fuel oxide. Therefore, the parasitic electrolysis of the aqueous electrolyte reduces the round trip efficiency of the secondary battery.
Self-discharge may result from impurities in the electrodes or reaction with the electrolyte. Typically, self-discharge from impurities in the electrodes is small (2-3% loss per month). The reaction of an active metal with water and/or O2 dissolved in the water, however, may be quite high (20-30% per month).
To compensate for these problems, metal-air batteries with aqueous electrolyte solutions are typically designed to contain a relatively high volume of electrolyte solution. Some cell designs even incorporate means for replenishing the electrolyte from an adjacent reservoir to maintain the electrolyte level. However, either approach adds to both the overall size of the cell, as well as the weight of the cell, without enhancing the cell performance (except to ensure that there is a significant volume of electrolyte solution to offset evaporation of the water or other solvent over time). Specifically, the cell performance is generally determined by the fuel characteristics, the electrode characteristics, the electrolyte characteristics, and the amount of electrode surface area available for reactions to take place. But the volume of electrolyte solution in the cell generally does not have a significant beneficial effect on cell performance, and thus generally only detracts from cell performance in terms of volumetric and weight based ratios (power to volume or weight, and energy to volume or weight). Also, an excessive volume of electrolyte may create a higher amount of spacing between the electrodes, which may increase ohmic resistance and detract from performance.
The use of non-aqueous systems for electrochemical cells has been suggested (see e.g., U.S. Pat. No. 5,827,602). In non-aqueous systems, the aqueous electrolyte may be replaced with an ionic liquid. Ionic liquids which contain a strong Lewis acid such as AlCl3, however, are known to liberate toxic gases when exposed to moisture.
The use of low or room temperature ionic liquid rather than an aqueous electrolyte in a metal-air electrochemical cell, are described in U.S. Provisional Application Ser. Nos. 61/383,510, filed Sep. 16, 2010; 61/355,081, filed Jun. 15, 2010; 61/334,047, filed May 12, 2010; 61/329,278, filed Apr. 29, 2010; 61/177,072, filed May 11, 2009, and 61/267,240, filed Dec. 7, 2009, and described in U.S. patent application Ser. No. 13/105,794, filed on May 11, 2011; Ser. No. 13/096,851, filed Apr. 28, 2011; Ser. No. 13/085,714, filed Apr. 13, 2011; and Ser. No. 12/776,962, filed May 10, 2010, the disclosures of each of which are incorporated herein by reference in their entirety. The use of a low or room temperature ionic liquid in the cell essentially eliminates the problems associated with evaporation of solvent from an electrolytic solution.
Room temperature ionic liquids have extremely low vapor pressures (some have vapor pressures that are essentially immeasurable under standard conditions) and thus experience little or no evaporation. Therefore, cells using low or room temperature ionic liquids as their ionically conductive media need not incorporate excessive volumes of solution in order to compensate for evaporation over time. Relatively small amounts of ionic liquid are sufficient to support the electrochemical reactions needed for cell operation, thereby reducing cell weight and volume and increasing power to volume/weight ratios. Also, other problems associated with solvents, such as hydrogen evolution in an aqueous solution, may be avoided.
Other problems exist in metal-air rechargeable batteries. For example, dendrites sometimes may form at the air electrode due to the diffusion of zinc and zinc hydroxides (e.g., zincate (Zn(OH)42−)) within the battery. Dendrites form from the growth of zinc (or other metals) in needle or branch-like structures into the electrolyte between the anode and the cathode. This type of growth during charging may cause the zinc to penetrate through the pores in a porous separator, which may cause short circuiting and battery failure when the zinc contacts the air electrode.
Catalysts or impurities from the air electrode may leach into the electrolyte, which may cause gassing at the metal electrode and degrade the performance of the battery. This leaching, if persistent over time, may eventually cause the air electrode to break down. It would be desirable to maintain the stability of the catalysts and any impurities so that they remain at the air electrode so that the air electrode remains intact over the life of the battery, and to the extent that any catalysts or impurities do become separated from the air electrode, to prevent them from being transported to the metal electrode to reduce the tendency of gassing at the metal electrode.
Conventional porous polymeric separators, as well as many solid polymer electrolytes, often allow zinc and zincate to diffuse between the metal electrode and the air electrode. Zincate ions are very soluble in alkaline electrolytes such as KOH, and will diffuse though a porous separator. In some cases with high zincate concentrations on the air electrode side of the separator, deposits of zinc oxide may occur once the solubility concentration of zincate is reached and the ZnO begins to precipitate. The solubility of zincate is closely linked to the OH− concentration, and one possible explanation for the deposition may be that OH− concentration varies during charge and discharge on the air electrode. Zincate deposition on the air electrode can cause failure of the battery.
The use of anion (or in general ion) exchange membranes in electrochemical power generation applications is known. Known anion exchange membranes used in electrochemical power generation applications are described in, for example, Gu, et al., “A Soluble and Highly Conductive Ionomer for High Performance Hydroxide Exchange Membrane Fuel Cells,” ACIE, pp. 1-4 (2009); Hwang, et al., “Preparation of anion-exchange membrane based on block copolymers . . . ”, J. Membrane Sci., 140, pp. 195-203 (1998); and Agel, et al., “Characterization and Use of Anionic Membranes for Alkaline Fuel Cells,” J. Power Sources, 101, pp. 267-274 (2001). These documents describe conventional membranes, such as chloromethylpolysulfone polymers, derivatized with certain functional groups (e.g., triarylphosphine or triethylamine).
Surrounding the cathode or anode with an ion exchange polymeric film also is known and described, for example, in U.S. Pat. Nos. 4,333,993 and 4,975,172. It has been proposed to incorporate ion exchange materials in metal/oxygen cells for a variety of purposes. As one example, U.S. Pat. No. 4,137,371 utilizes an ion-exchanging membrane as a zincate restricting membrane to prevent poisoning of the electrochemically active material in a zinc/oxygen cell. The ion-exchanging membrane is joined directly to the oxygen electrode and is positioned between the porous layer of this electrode and the zinc electrode. U.S. Pat. No. 3,514,336 describes an electrochemical cell which utilizes an ion exchange resin matrix having macroporous channels containing a free electrolyte in the channels that is disposed between the anode and cathode. When used in a fuel cell, such matrix is said to act as mixed current carriers so that the resins cannot dehydrate as a result of endosmotic transport, thus providing an electrolyte which remains homogenous. A further use of ion-exchange materials is described in U.S. Pat. No. 3,097,115 wherein natural and synthetic zeolites are utilized to form electrodes for fuel cells. The electrodes are formed, by, in general, shaping the electrode as desired, ion exchanging the naturally occurring ions from the zeolite with the desired activating metallic exchange properties. U.S. Pat. No. 3,097,116 discloses forming an electrode structure by bonding the heat stabilized, ion exchange zeolite to a gas diffusion membrane, which membrane may be either hydrophilic or hydrophobic.
U.S. Pat. No. 5,798,180 discloses a battery separator (or coating on an electrode) comprised of a polyaromatic ether, such as sulfonated poly(2,6-dimethyl-1,4-phenylene oxide). Other known battery separators include Nafion™, commercially available from DuPont, Wilmington, Del., (a sulfonated polytetrafluoroethylene), fluorosulfonated Teflon®, polyethylene separators available from Daramic, Charlotte, N.C., and polypropylene and polyethylene separators available from Celgard, Charlotte, N.C. Metal-air batteries, electrochemical devices, and/or fuel cells containing ion-exchange membranes as part of an electrode, or as a battery separator also are disclosed in U.S. Patent Application Publication Nos. 2004/0157101, 2008/0008937, 2010/0137460, and 2011/0027666, the disclosures of which are incorporated by reference herein in their entireties. There is a need to provide improved ion exchange materials for use in metal-air rechargeable batteries.